In the thermal power system, the temperature of steam supplied to the steam turbine is becoming increasingly higher in order to provide the higher electricity production efficiency. Accordingly, the turbine materials must meet the more strict requirements for the high temperature properties. For those years, numerous heat resisting steels have been developed, and have been used to meet such requirements. Some of the heat resisting steels are known as having the good resistance to the comparatively high temperatures, as proposed in Japanese patent applications now published under No. Heisei 4 (1992)-147948 and No. Heisei 8 (1996)-3697, respectively.
For the high Cr (chromium) ferrite-type heat resisting steel, however, it is found that the creep strength that such steels possess will be reduced remarkably, when they are used at 650° C. for a long period of time. To avoid that such reduction in the creep strength occurs at 650° C., the maximum temperature at which the steels can be used is actually limited to less than about 620° C. Therefore, demands arise for turbine materials that can be used at 650° C. for a long period of time, without causing the remarkable reduction in the creep strength.
The present invention is based on the above observations, and it is an object of the present invention to provide novel heat resisting steels that can maintain their good the high temperature properties and permanence properties etc, by controlling the remarkable reduction in the creep strength that may occur when the steels are used at high temperatures, such at least 650° C., for the long period of time.